Adrenoceptor-mediated changes of excitation and contraction in ventricular heart muscle from guinea-pigs and rabbits.

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1. The influence of alpha-adrenoceptor stimulation on mechanical and electrophysiological parameters was investigated in ventricular preparations from guinea-pigs and rabbits. Action potential and force of contraction were measured in papillary muscles and ionic currents were measured in isolated myocytes. 2. The effects of alpha-adrenoceptor stimulation were compared with those of beta-adrenoceptor stimulation. 3. In the guinea-pig the stimulation of alpha-adrenoceptors caused a small increase in the force of contraction (less than 10% of the response to beta-adrenoceptor stimulation) which was not accompanied by any increase of the slow calcium inward current. beta-Adrenoceptor stimulation produced large increases in both force of contraction and slow inward calcium current. The noradrenaline-induced increase in the slow inward calcium current was not affected by phentolamine. 4. In the rabbit, alpha-adrenoceptor stimulation produced large increases in the force of contraction (about two thirds of those seen in response to beta-adrenoceptor stimulation). Whereas beta-adrenoceptor stimulation also produced large increases in both maximal upstroke velocity of slow-response action potentials and slow inward calcium current, there was almost no change of both parameters in response to alpha-adrenoceptor stimulation. 5. We conclude that, first, the contribution of alpha-adrenoceptors to adrenoceptor-mediated changes of force of contraction is minimal in the guinea-pig ventricle, and second, the pronounced changes of force of contraction in the rabbit ventricle in response to alpha-adrenoceptor stimulation are unrelated to changes in the slow inward calcium current.

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